1. Field of the Invention
The present invention relates to a cathode-ray tube for use in, for example, a color television, a color display apparatus or the like.
2. Description of the Prior Art
A resolution characteristic of a color cathode-ray tube generally depends largely upon the size and shape of the beam spot. That is, if the diameter of the beam spot generated on a fluorescent screen by collision of an electron beam thereon is neither small nor substantially perfectly circular, then a satisfactory resolution characteristic cannot be obtained. A cathode-ray tube in which a static quadrupole lens is provided in order to improve a focus at the periphery of the screen is disclosed in U.S. applications Nos. 974714 filed Nov. 12, 1992 and 976391 filed Nov. 13, 1992.
However, since an orbit of the electron beam from an electron gun to the fluorescent screen becomes long in response to a deflection angle of the electron gun, if a focus voltage is maintained so that a small and perfectly circular beam spot should be obtained at the center of the fluorescent screen, then a peripheral portion of the fluorescent screen is brought in an over-focus state, with the result that the beam spot having a small diameter cannot be obtained at the peripheral portion thereof and hence satisfactory resolution cannot be obtained.
Therefore, there has been recently proposed an electron gun for the cathode-ray tube adopting a so-called dynamic focus system in which the focus voltage is increased and a main-lens action is lowered in response to an increase of the deflection angle of the electron beam (for example, Shoji Shirai, Masakazu Fukushima et al.: Quadrupole Lens for Dynamic Focus and Astigmatism Control in an Elliptical Aperture Lens Gun, Proceeding SID 87 DIGEST P162-165, and Japanese Laid-open Patent Publication NO. 93135/1991).
However, such prior art encounters the following problems.
That is, in case of the above Proceeding SID 87 Digest, for example, a dynamic quadrupole lens for lowering the main-lens action is formed on the side of a cathode relative to the main lens, so that a distance between a deflection yoke, which is a portion where a spot deformation itself is caused, and a correction point becomes long. Accordingly, a dynamic focus voltage upon the correction becomes high to increase load on a correction circuit, and an aspect ratio of the beam spot upon a dynamic correction becomes aggravated (a length in a lateral direction becomes long and a length in a longitudinal direction becomes short).
On the other hand, in the above Japanese Laid-open Patent Publication, it is proposed to improve the aspect ratio of the spot upon the correction by combining two quadrupole lenses. However, since the quadrupole lenses are formed on the cathode side relative to the main lens lengthen the beam spot, the above-mentioned problem cannot be sufficiently improved. Even in this case, the main effect thereof is to in the longitudinal direction, so that the length in the lateral direction thereof is prevented from becoming short. Therefore, as a result, there is then the disadvantage that the resolution is not improved.